Many commercially important polypeptides are recombinantly produced in cells grown in culture. One of the limits to growing cells in culture is the decreased viability of the cells over time, which is partially rectified by the addition of an extracellular signal, such as a growth factor. However, while this extra step improves cell viability, it adds significant costs to the production of the desired recombinant polypeptide.
Cellular responses to extracellular signals such as growth factors are controlled, in part, by post-translational modifications of polypeptides expressed on the cell surface and/or in the cell cytoplasm. Highly dependable regulation of post-translational modifications is important in maintaining the integrity of normal cell signaling. Kinases, which add phosphates to substrates, and phosphatases, which remove phosphates from phosphorylated substrates, are two large classes of enzymes regulating post-translational modifications of cellular machinery that are frequently mutated in human disease (Schlessinger (2000) Cell, 103(2):211–25; Fischer et al., (1991) Science, 253:401–6; Hunter (1998–99) Harvey Lect., 94:81–119).
The study of phosphorylation regulation has led to the identification of numerous non-proteinaceous compounds that inhibit or activate enzymes. One such compound, vanadate, has been identified as a molecule which has a specific inhibitory activity toward tyrosine phosphatases. Vanadate ions have been shown to also act as insulin mimics in animal cells by an as yet unknown mechanism (Pandey et al., (1999) Biochemistry, 38:14667–14675; Posner et al., (1994) J. Biol. Chem., 269:4596–4604; Huyer et al., (1997) J. Biol. Chem., 272:843–851; Scheving et al., (1999) Am. J. Physiol., 277:C572–9). Vanadate has also been shown to inhibit apoptosis in a mammalian cell line subjected to serum withdrawal in an insulin receptor independent manner (Lee-Kwon et al., (1998) Biochemistry, 37:15747–15757). However, vanadate has been shown to have toxicity in humans and other mammals when tested for use as an insulin substitute (see generally, Sekar et al., (1996) Crit. Rev. Biochem. Mol. Biol., 31(5):339–359). Nobuhara et al. (U.S. Pat. No. 4,680,261) added vanadate, among other metals, to non-recombinant cell cultures producing endogenous interferons.
Thus, there is a need in the art for methods of improving the cell viability of recombinant cell cultures to reduce cell death, to reduce the dependence on growth factors and to increase recombinant polypeptide production while minimizing costs. The invention fulfills this need by providing a simple, easy and inexpensive way of increasing recombinant cell viability and reducing the requirement for growth factors by cultured animal cells.